Why everything shifts
The pregnancy hormone hCG is structurally similar to TSH and stimulates the thyroid as well. As a result, TSH often drops below the usual reference range in the first trimester – that is not an error but normal.
At the same time, requirements rise: more binding protein in the blood, a larger volume of distribution and a baby that needs supplying too. For women with a known underactive thyroid this almost always means a dose increase.
At a glance
- Requirements: the levothyroxine dose often rises by 25 to 50 per cent
- Monitoring: roughly every four to six weeks until mid-pregnancy
- Reference ranges: TSH is physiologically lower in the first trimester
- Iodine: around 230 micrograms a day
- After the birth: bear postpartum thyroiditis in mind
- Appointment: book online or call us on +43 1 5333 654
If you are already taking levothyroxine
Please get in touch as soon as your pregnancy test is positive – not only at your first Mutter-Kind-Pass (Austrian maternity record) appointment. The dose is often raised in the very first weeks, frequently by around a quarter to a third.
After that we check every four to six weeks until the value is stable within the target range, and less often thereafter. After the birth, requirements usually return to the previous level.
Hyperthyroidism in pregnancy
An overactive thyroid needs special attention during pregnancy. The choice of medication differs from the approach outside pregnancy, and the dose is kept as low as possible.
This must be distinguished from so-called gestational hyperthyroidism: a temporary, hCG-related overactivity in the first trimester, often accompanied by severe nausea. It resolves on its own and is generally not treated with antithyroid drugs.
After the birth: postpartum thyroiditis
Around five to ten per cent of women develop a temporary inflammation of the thyroid in the first months after giving birth. A two-phase course is typical: first a few weeks of overactivity, then a phase of underactivity.
Because exhaustion, hair loss and low mood occur after a birth anyway, this is often overlooked. If symptoms persist, it is therefore worth looking at the values – especially if TPO antibodies are known.
Frequently asked questions
My TSH is suddenly very low – is that dangerous?
In the first trimester a low TSH is common and usually harmless, because hCG also stimulates the thyroid. What matters is the fT4 and whether you have symptoms. We therefore always assess the value in context, never in isolation.
Are thyroid values checked as part of the Mutter-Kind-Pass?
Routine testing is not part of the Mutter-Kind-Pass (Austrian maternity record) programme. However, if you have a known thyroid condition, confirmed antibodies, a family history, previous miscarriages or corresponding symptoms, checking is clearly worthwhile.
Will levothyroxine harm my baby?
No. Correctly dosed, it provides exactly the supply your baby needs. What is dangerous is an untreated underactive thyroid, not its treatment.